Electrical earth borehole logging is well known and various devices and various techniques have been described for this purpose. Broadly speaking, there are two categories of devices used in electrical logging devices. In the first category, a transmitter (such as a guarded electrode) is uses in conjunction with a diffuse return electrode (such as the tool body). A measured electric current flows in a circuit that connects a voltage source to the transmitter, through the earth formation to the return electrode and back to the voltage source in the tool. In the guarded electrode, a second or center electrode is fully or at least partially surrounded by a guard electrode. Provided both electrodes are kept at the same potential, a current flowing through the center electrode is focused into the earth formation by means of the guard electrode. Generally, the center electrode current is several orders of magnitude smaller than the guard current.
In inductive measuring tools, an antenna within the measuring instrument induces a current flow within the earth formation. The magnitude of the induced current is detected using either the same antenna or a separate receiver antenna. The present disclosure belongs to the second category.
Calibration is a critical procedure before making measurements with any electrical logging tool because it allows correlations and corrections of the realistic tool responses in order to match modeled tool responses in ideal situations which are necessary for accurate interpretation of the electrical logs. With tools in the second category, calibration errors may occur due to variations and uncertainties in ground resistivity at different locations where the calibration processes take place. The ground effect becomes more severe when the depths of investigation of the induction tools increase. Therefore, in order to reduce the ground effect when calibrating deep-reading induction tools, particular procedures are usually taken which involve lifting the entire deep-reading induction tool to a significant height (usually over 20 feet) above ground and then performing the calibration measurements (the “air-hang” method). This method is not only expensive but unsafe as well. This disclosure addresses the minimization and mitigation of the errors induced by the ground effect without using the “air-hang” method.